Atomistic Simulation and Theory of Nanoporous Carbons and Nanostructures

International audienc

Freezing point depression and freeze-thaw damage by nano-fuidic salt trapping

A remarkable variety of organisms and wet materials are able to endure temperatures far below the freezing point of bulk water. Cryo-tolerance in biology is usually attributed to "anti-freeze" proteins, and yet massive supercooling ($< -40^\circ$C) is also possible in porous media containing only simple aqueous electrolytes. For concrete pavements, the common wisdom is that freeze-thaw damage results from the expansion of water upon freezing, but this cannot explain the large pressures ($> 10$~MPa) required to damage concrete, the observed correlation between pavement damage and de-icing salts, or the damage of cement paste loaded with benzene (which contracts upon freezing). In this Letter, we propose a different mechanism -- nanofluidic salt trapping -- which can explain the observations, using simple mathematical models of dissolved ions confined to thin liquid films between growing ice and charged surfaces. Although trapped salt lowers the freezing point, ice nucleation in charged pores causes enormous disjoining pressures via the rejected ions, until their removal by precipitation or surface adsorption at a lower temperatures releases the pressure and allows complete freezing. The theory is able to predict the non-monotonic salt-concentration dependence of freeze-thaw damage in concreter and provides a general framework to understand the origins of cryo-tolerance.Comment: 5 figure

Multiscale Poromechanics of Wet Cement Paste

Capillary effects such as imbibition-drying cycles impact the mechanics of granular systems over time. A multiscale poromechanics framework was applied to cement paste, that is the most common building material, experiencing broad humidity variations over the lifetime of infrastructure. First, the liquid density distribution at intermediate to high relative humidities is obtained using a lattice gas density functional method together with a realistic nano-granular model of cement hydrates. The calculated adsorption/desorption isotherms and pore size distributions are discussed and compare well to nitrogen and water experiments. The standard method for pore size distribution determination from desorption data is evaluated. Then, the integration of the Korteweg liquid stress field around each cement hydrate particle provided the capillary forces at the nanoscale. The cement mesoscale structure was relaxed under the action of the capillary forces. Local irreversible deformations of the cement nano-grains assembly were identified due to liquid-solid interactions. The spatial correlations of the nonaffine displacements extend to a few tens of nm. Finally, the Love-Weber method provided the homogenized liquid stress at the micronscale. The homogenization length coincided with the spatial correlation length nonaffine displacements. Our results on the solid response to capillary stress field suggest that the micronscale texture is not affected by mild drying, while local irreversible deformations still occur. These results pave the way towards understanding capillary phenomena induced stresses in heterogeneous porous media ranging from construction materials, hydrogels to living systems.Comment: 6 figures in main text, 4 figures in the SI appendi

Molecular Simulations of Supercritical Fluid Permeation through Disordered Microporous Carbons

International audienceFluid transport through microporous carbon-based materials is inherent in numerous applications, ranging from gas separation by carbon molecular sieves to natural gas production from coal seams and gas shales. The present study investigates the steady-state permeation of supercritical methane in response to a constant cross-membrane pressure drop. We performed dual control volume grand canonical molecular dynamics (DCV-GCMD) simulations to mimic the conditions of actual permeation experiments. To overcome arbitrary assumptions regarding the investigated porous structures, the membranes were modeled after the CS1000a and CS1000 molecular models, which are representative of real microporous carbon materials. When adsorption-induced molecular trapping (AIMT) mechanisms are negligible, we show that the permeability of the microporous material, although not significantly sensitive to the pressure gradient, monotonically decreases with temperature and reservoir pressures, consistent with diffusion theory. However, when AIMT occurs, the permeability increases with temperature in agreement with experimental data found in the literature

Freezing point depression and freeze-thaw damage by nanofluidic salt trapping

A remarkable variety of organisms and wet materials are able to endure temperatures far below the freezing point of bulk water. Cryotolerance in biology is usually attributed to “antifreeze” proteins, and yet massive supercooling (10 MPa) required to damage concrete, the observed correlation between pavement damage and deicing salts, or the FT damage of cement paste loaded with benzene (which contracts upon freezing). In this work, we propose a different mechanism—nanofluidic salt trapping—which can explain the observations, using simple mathematical models of dissolved ions confined between growing ice and charged pore surfaces. When the transport time scale for ions through charged pore space is prolonged, ice formation in confined pores causes enormous disjoining pressures via the ions rejected from the ice core, until their removal by precipitation or surface adsorption at lower temperatures releases the pressure and allows complete freezing. The theory is able to predict the nonmonotonic salt-concentration dependence of FT damage in concrete and provides some hint to better understand the origins of cryotolerance from a physical chemistry perspective

Docking 90 Sr radionuclide in cement: An atomistic modeling study

a b s t r a c t Cementitious materials are considered to be a waste form for the ultimate disposal of radioactive materials in geological repositories. We investigated by means of atomistic simulations the encapsulation of strontium-90, an important radionuclide, in calcium-silicate-hydrate (C-S-H) and its crystalline analog, the 9 Å-tobermorite. C-S-H is the major binding phase of cement. Strontium was shown to energetically favor substituting calcium in the interlayer sites in C-S-H and 9 Å-tobermorite with the trend more pronounced in the latter. The integrity of the silicate chains in both cementitious waste forms were not affected by strontium substitution within the time span of molecular dynamics simulation. Finally, we observed a limited degradation of the mechanical properties in the strontium-containing cementitious waste form with the increasing strontium concentration. These results suggest the cement hydrate as a good candidate for immobilizing radioactive strontium

Ion specificity of confined ion-water structuring and nanoscale surface forces in clays

Ion specificity and related Hofmeister effects, ubiquitous in aqueous systems, can have spectacular consequences in hydrated clays, where ion-specific nanoscale surface forces can determine large scale cohesive, swelling and shrinkage behaviors of soil and sediments. We have used a semi-atomistic computational approach and examined sodium, calcium and aluminum counterions confined with water between charged surfaces representative of clay materials, to show that ion-water structuring in nanoscale confinement is at the origin of surface forces between clay particles which are intrinsically ion-specific. When charged surfaces strongly confine ions and water, the amplitude and oscillations of the net pressure naturally emerge from the interplay of electrostatics and steric effects, which can not be captured by existing theories. Increasing confinement and surface charge densities promote ion-water structures that increasingly deviate from the ions' bulk hydration shells, being strongly anisotropic and persistent, and self-organizing into optimized, nearly solid-like assemblies where hardly any free water is left. In these conditions, strongly attractive interactions can prevail between charged surfaces, due to the dramatically reduced dielectric screening of water and the highly organized water-ion structures. By unravelling the ion-specific nature of these nanoscale interactions, we provide evidence that ion-specific solvation structures determined by confinement are at the origin of ion specificity in clays and potentially a broader range of confined aqueous systems.Comment: Main text: 14 pages and 6 figures. Supporting information: 5 figures. Submitted to The Journal of Physical Chemistry

Capillary stress and structural relaxation in moist granular materials

We propose a theoretical framework to calculate capillary stresses in complex mesoporous materials, such as moist sand, nanoporous hydrates, and drying colloidal films. Molecular simulations are mapped onto a phase-field model of the liquid-vapor mixture, whose inhomogeneous stress tensor is integrated over Voronoi polyhedra in order to calculate equal and opposite forces between each pair of neighboring grains. The method is illustrated by simulations of moisture-induced forces in small clusters and random packings of spherical grains using lattice-gas Density Functional Theory. For a nano-granular model of cement hydrates, this approach reproduces the hysteretic water sorption/desorption isotherms and predicts drying shrinkage strain isotherm in good agreement with experiments. We show that capillary stress is an effective mechanism for internal stress relaxation in colloidal random packings, which contributes to the extraordinary durability of cement paste.Comment: 4 figure